For RF PAs used in wireless communication, linearity may be very important. Generally, linearity may be a measure of how linear the RF signal output of an RF PA is as the RF input signal is increased. In other words, linearity may refer to the gain (sometimes referred to as AM-AM distortion wherein AM may refer to amplitude modulation) and phase shift (sometimes referred to as AM-PM distortion wherein PM may refer to phase modulation) of the RF PA, and it may be desirable for the gain and phase to be consistent over a range of RF signal inputs or outputs such that the gain and phase of the RF PA at one signal input is approximately the same as the gain and phase of the RF PA at another signal input. One of the key measurements of the RF PA linearity may be the ACPR (Adjacent Channel Power Ratio), which may be the measure of the ratio between the total adjacent channel power (sometimes referred to as an intermodulation signal) to the main channel's power (sometimes referred to as a useful signal).
Existing RF PAs may include a bias circuit designed to bias the power amplifier to improve linearity. The bias circuit may use a bipolar transistor, such as heterojunction bipolar transistor (HBT), a field effect transistor such as a metal-oxide-semiconductor field effect transistor (MOSFET), a metal-semiconductor field effect transistor (MESFET), a pseudomorphic high-electron mobility transistor (PHEMT), or a combination of the above such as a bipolar field effect transistor (BiFET) and/or a bipolar high-electron-mobility transistor (BiHEMT). However, regardless of the bias circuit, existing RF PAs may still exhibit non-linear distortion in the output signal of the RF PA as the RF input signal of the RF PA increases.